Why did the Japanese attack Pearl Harbor?

Japan
attacked Pearl Harbor because it wanted to have an empire in East Asia and it feared that the
United States would prevent that from happening.

Japan had, at least since
the Russo-Japanese War of 1905, thought of itself as a major power.  It felt that it should have
an empire like those of the other major powers.  However, it was prevented from getting an
empire because much of the area around Japan was already colonized by other
countries. 

Oncestarted in Europe, this changed to some degree.  The Germans,
who were allied with the Japanese, had defeated France and the Netherlands.  This meant that
Japan could hope to take French colonies in Indochina and Dutch colonies in Indonesia.  The
Germans were fighting the British, and the Japanese could also hope to take British colonies in
the area.  The main problem was that the US did not want that to happen. 

For
example, when Japan took French Indochina, the US stopped selling oil and scrap iron (we were
Japans major source for these important commodities) to Japan.  Japan calculated that it would
run out of oil in about two years.  Therefore, it needed to take the oilfields in Indonesia or
else it would run out of oil and be at the mercy of any country that wanted to push it
around.

Japan decided to go ahead and take an empire.  But first it needed to
make sure the US could not interfere.  Therefore, it attacked Pearl Harbor to try to destroy the
US fleet and make it impossible for the US to interfere.